If most displays are looked at, the observer can only see a flat pixel plane. However, it is desired to look at a display and to see the objects presented on it in a three-dimensional view that is in a realistic manner. Various attempts have been made in this respect in order to find a satisfactory solution for this problem. One solution is volumetric displays, which can generate a three-dimensional image, but which require a complicated apparatus. Further, autostereoscopic displays with lenticular arrays are known from prior art documents. However, these display devices do not represent a true three-dimensional image. At present, the best solution of generating a three-dimensional image is to take advantage of holography. Holography provides a true three-dimensional image with a desired depth accommodation (movement parallax etc.) and high resolution.
EP 1 467 263 A1, for example, discloses a holographic display device for reconstructing a three-dimensional scene. That display device comprises a reflective light modulator, a beam splitter for projecting a hologram, an aperture stop, a field lens and a collimator lens. The hologram is generated on a computer using three-dimensional object information and then presented on the light modulator. The light modulator is illuminated with light emitted by a light source and projected through the beam splitter, such that a three-dimensional scene is reconstructed. The reconstructed scene is generated near the field lens. However, the dimension of the scene is adversely limited by the size of the field lens. Further, the observer of the reconstructed scene only has limited freedom of movement, because a tracking facility for an observer eye has not been proposed. Moreover, periodical repetitions of the diffraction orders appear in the Fourier plane.
Spatial light modulators (SLM) used accordingly modulate the phase and/or amplitude of light. Generally, such a light modulator has more than one million modulation elements, which are referred to as pixels. In order to achieve a high resolution, and thus to get a large visibility region and a large reconstructed scene, the light modulator is desired to have a large number of pixels. Because the trend towards miniaturisation is progressing at a fast pace, the light modulators are constantly required to become smaller. As the pixel size can hardly be reduced further, however, it has hitherto not been possible from a technological point of view, or it is at least very difficult, to achieve a large number of pixels on the light modulator with discretely controllable optical properties.
One possibility of enlarging the visibility region is described in “Electro-holographic display using 15 mega pixels LCD”, Proc. of SPIE Volume 2652, pp. 15 to 23, K. Maeno et al. In that display, five light modulators are horizontally arranged, whereby the total pixel number of the display becomes about 15 million pixels. Such a high pixel number of the display allows the reconstructed scene and the visibility region to be enlarged, so that an observer can watch the reconstructed scene with both eyes, i.e. binocularly. However, the disadvantage of such a display is that the light modulators, or more precisely the light emitted by the light modulators exhibits mutual coherence, so that the coherent light is superimposed and disturbing interference effects such as speckles occur. Further, it is difficult to make such a display, because the multiple mirrors needed for beam guidance must be precisely aligned.
Another way of enlarging the visibility region is described in the yet unpublished document DE 10 2006 024 356.0. The projection device disclosed therein comprises a two-dimensional light modulator device in a scanning system, where the light scans one after another multiple one-dimensional pixel arrangements of the light modulator device with the help of a scanning element. A wave front modulated with the help of the light modulator device is imaged into a virtual visibility region or on to a screen. In order for the observer to be able to watch the reconstructed scene even if he moves, it is necessary to track the virtual visibility region to the respective observer eye. Although it is generally possible to watch the reconstructed scene binocularly, this is difficult to achieve.